The pressure in a supercritical freezing cycle in which CO2 (carbon dioxide) is used as the coolant rises to a level approximately 10 times as high as the pressure in a freezing cycle in which a Freon coolant is used. For this reason, as the coolant is compressed inside a cylinder bore, the temperature of the discharged coolant, too, increases due to the raised discharge pressure, and since this lowers the viscosity of the lubricating oil, the lubrication at sliding portions becomes poor, which leads to a problem of poor durability of the sliding portions. Further problems may arise if the lubricating oil becomes degraded due to the heat or if the lowered viscosity causes sliding portions to seize.
This issue is addressed in the related art with a structure disclosed in Japanese Unexamined Patent Publication No. 2000-18154, in which degradation of the lubricating oil and the occurrence of a seize are prevented by forming a continuous hollow portion around a plurality of cylinder bores formed at a cylinder block, supplying a feedback coolant (a cooling medium), which is taken in via a suction port, into the hollow portion and preventing the temperature inside the bores from rising to an excessive extent during the compression phase through active heat exchange of the heat inside the individual cylinder bores and the feedback coolant.
However, while the structure described above is advantageous in that the cylinder bores are cooled to a sufficient degree, it simply cools down the lubricating oil by cooling the cylinder bores. Thus, when the lubricating oil mixed in the discharged coolant is separated and is directly returned to the crankcase, the temperature of the returning lubricating oil will be high. As a result, good lubrication will not be achieved in the crankcase due to the presence of a significant quantity of lubricating oil with lowered viscosity, which gives rise to a concern for lowered durability of the sliding portions. In addition, there is a concern in that the cylinder bores with the hollow portion formed around them in the structure described above may become deformed readily as the descending loads of the pistons are applied.
Accordingly, a main object of the present invention is to provide a reciprocating compressor that achieves improved durability in the sliding portions by effectively cooling the lubricating oil inside the compressor and assuring good lubrication at the sliding portions. Further objects of the present invention are to assure effective cooling of the cylinder bores and the pistons and to prevent deformation of the bores, which tends to occur readily when a hollow portion is formed around the cylinder bores.